Conventionally, a nitridosilicate-based compound containing as major elements at least (1) an alkaline-earth metal element M (where M is at least one element selected from Mg, Ca, Sr, and Ba), (2) silicon, and (3) nitrogen, and a nitridosilicate-based compound containing as major elements at least (1) a rare earth element Ln (where Ln is at least one element selected from rare earth elements of atomic numbers 21, 39, and 57-71), (2) silicon, and (3) nitrogen are known.
Examples of the above-mentioned nitridosilicate-based compound include Sr2Si5N8, Ba2Si5N8 (see Patent documents 1-3, and Non-patent document 1 described below), BaSi7N10 (see Patent documents 1-3 described below), SrSiAl2O3N2, Sr2Si4AlON7, La3Si6N11 (see Patent document 4 described below), Eu2Si5N8, EuYbSi4N7 (see Non-patent document 2 described below), (Ba, Eu)2Si5N8 (see Non-patent document 3 described below), Ce4(Si4O4N6)O, Sr3Ce10Si18Al12O18N36 (see Non-patent document 4 described below), CaSiN2 (see Non-patent document 5 described below), and the like. In the present specification, SIALON (see Patent document 5 described below) represented by a general formula: Mp/2Si12−p−qAlp+qOqN16−q (where M is Ca or Ca combined with Sr; q is 0 to 2.5; and p is 1.5 to 3) is excluded.
It is known that the above-mentioned CaSiN2 becomes a CaSiN2:Eu2+ phosphor emitting red light having an emission peak in the vicinity of 630 nm by being activated with Eu2+ ions being a luminescent center. The following also is known: the excitation spectrum of the above-mentioned phosphor has a peak in the vicinity of 370 nm, and although the phosphor does not emit red light with a high intensity at excitation of blue light in a range of 440 nm to less than 500 nm, it emits red light with a strong output at near-ultraviolet light excitation in a range of 330 to 420 nm. Therefore, the application to a light-emitting apparatus using a light-emitting element emitting near-ultraviolet light as an excitation source is considered to be promising (see Non-patent document 5 described below).
Furthermore, the following also is known: the above-mentioned nitridosilicate-based compound can be applied as a phosphor material as well as a ceramic material, and the above-mentioned nitridosilicate-based compound, for example, containing Eu2+ ions and Ce3+ ions becomes a high-efficiency phosphor (see Patent documents 1 to 6 described below).
Furthermore, it also is known that the above-mentioned high-efficiency phosphor composed of a nitridosilicate-based compound is suitable as an LED light source, since it is excited with near-ultraviolet light to blue light, and emits visible light of blue, green, yellow, orange, or red (see Patent documents 1 to 3, and Non-patent document 5 described below).
Conventionally, in order to produce the above-mentioned nitridosilicate-based compound, a production method has been used, in which alkaline-earth metal (metal Ca, metal Sr, metal Ba, etc.) or a nitride of alkaline-earth metal (Ca3N2, Sr3N2, Ba3N2, etc.) is used as a supply source of alkaline-earth metal, and rare earth metal (metal La, metal Ce, metal Eu, etc.) is used as a supply source of a rare earth element, without using a reducing agent (solid-state carbon, etc. described below) excluding alkaline-earth metal and rare earth metal (see Patent documents 1-6, and Non-patent documents 1-4).
On the other hand, conventionally, the use of a phosphor of a nitridosilicate-based compound produced by such a production method in a light-emitting apparatus such as an LED light source has been studied.
(Patent document 1) JP2003-515655A
(Patent document 2) JP2003-515665A
(Patent document 3) JP2002-322474A
(Patent document 4) JP2003-206481A
(Patent document 5) JP2003-203504A
(Patent document 6) JP2003-124527A
(Non-patent document 1) T. Schlieper et al., Z. an org. allg. Chem., Vol. 621, (1995), pages 1380-1384
(Non-patent document 2) H. Huppertz and W. Schnick, Acta Cryst., Vol. 53, (1997), pages 1751-1753
(Non-patent document 3) H. A. Hoppe et al., J. Phys. Chem. Solids, Vol. 61 (2000), pages 2001-2006
(Non-patent document 4) W. Schnick, Int. J. Inorg. Mater., Vol. 3 (2001), pages 1267-1272
(Non-patent document 5) K. Ueda et al., Extended Abstracts of 71st Meeting of The Japan Society of Electrochemistry (2004), page 75
However, according to the conventional method for producing a nitridosilicate-based compound, above all, a method for producing a highly nitrided nitridosilicate-based compound (e.g., M2Si5N8, MSi7N10, M2Si4AlON7, MSiN2 (where M is at least one element selected from Mg, Ca, Sr, and Ba), etc.) with a small number of oxygen atoms, in particular, a nitridosilicate-based compound containing substantially no oxygen component, alkaline-earth metal or rare earth metal, which is chemically unstable and has the danger of ignition, or a nitride of alkaline-earth metal or rare earth nitride, which is difficult to obtain, is very expensive, and is difficult to handle, is used as a supply source of alkaline-earth metal or a rare earth element. Therefore, the above-mentioned method has the following problems, which makes it very difficult to industrially produce a nitridosilicate-based compound.
(1) It is difficult to mass-produce a nitridosilicate-based compound.
(2) It is difficult to produce a high purity compound of high quality with satisfactory reproducibility.
(3) It is difficult to provide an inexpensive compound.
Since the conventional production method has such problems, the conventional nitridosilicate-based compound has the following problems: (1) low purity due to the presence of a large amount of impurity oxygen; (2) low material performance such as low emission performance of a phosphor caused by the low purity; (3) high cost; and the like. For example, the conventional light-emitting apparatus using the conventional nitridosilicate-based phosphor as a light-emitting source has the following problems: (1) low luminous flux and brightness; (2) high cost; and the like.